The present invention relates to a riffle distributor assembly for a fossil fuel fired combustion arrangement and, more particularly, to a riffle distributor assembly for a fossil fuel fired combustion arrangement of the type having fossil fuel delivery systems that deliver pulverized coal to coal fired steam generators.
Coal fired furnaces are typically provided with a plurality of ducts or pipes through which pulverized coal and air is directed to a plurality of fuel-air admission assemblies arrayed in respective vertically extending windboxes. The windboxes are disposed in one or more walls of the furnace and each introduces coal and air into the furnace.
Pulverized coal firing is favored over other methods of burning coal because pulverized coal burns like gas and, therefore, fires are easily lighted and controlled. Such systems may include one or more pulverizers, also referred to as mills, that are used to grind or comminute the fuel or, alternatively, may not include any pulverizer because a supply of pulverized coal available.
The pipes directing the coal to the respective windboxes are large and cumbersome. Typically the pipes are provided with large couplings or bolted flanges to couple the end abutting axially adjacent portions together. The normal nozzle assembly requires regular maintenance because the pulverized coal has a severe erosive effect. A typical pulverizer will move between 7 and 50 tons of coal every hour. The coal typically moves at a velocity of 75–90 feet per second within the fuel transport pipe.
A typical coal distribution system includes a number of distributors intended to split the flow of air and pulverized coal into two discrete pipes. It is desired that the distributors take the homogeneous mixture of pulverized coal and deliver identical quantities of that homogeneous flow to each of the two discrete pipes. Each of these distributors is a Y-shaped duct. Each of these Y-shaped ducts has an inlet and two outlets. U.S. Pat. No. 5,934,205 to Gordon et al discloses a Y-shaped distributor body and a splitter disposed in the distributor body for dividing a flow of pulverized coal between first and second outlets.
In connection with the feed of pulverized coal to the feed burner nozzles of a combustion chamber, U.S. Pat. No. 6,055,914 to Wark notes that an exhauster fan first throws the coal radially into a primary discharge chute and that the flow of coal/air leaving the exhauster fan is uneven, whereby the coal/air flow to the burners tends to be light on one side or wall of the chute and heavy on the other side or wall of the chute in terms of both particle size and distribution.
U.S. Pat. No. 6,055,914 to Wark describes a prior art solution which involves providing “riffle boxes” in the chute between the fan and the burners. A riffle box is a series of vertical, spaced plates separated by angled separator bars with alternating orientation from plate to plate and U.S. Pat. No. 6,055,914 to Wark notes that, in accordance with one theory, it is believed that the separator bars on one plate will deflect the coal in one direction, while the separator bars on adjacent plates will deflect the coal in the opposite direction, thereby splitting and redistributing the flow for a more homogeneous mixture. It is further noted in this reference that the typical arrangement is to provide a series of riffle boxes, with a first riffle box splitting the flow like a “Y” into two chute branches, and a subsequent riffle box on each of the first two branches splitting the flow again into a total of four chutes. Each chute typically fuels one of four corner-mounted burners in a tangentially-fired combustion chamber.
U.S. Pat. No. 6,055,914 to Wark notes that the riffle boxes have proven ineffective in providing a more homogeneous mixture to the burners, and the coal/air flow reaching the four combustion chamber burners differs significantly from burner to burner. The reference cites several problems which result from a riffle box arrangement: too lean a mixture at a burner can create NOX; oversized particles and inefficient burning create LOI (loss on ignition) contamination of the ash byproduct and reduced combustion efficiency; and, perhaps most importantly, the out-of-balance burner flow distorts the combustion chamber fireball from the ideal spherical shape to an undesirable elliptical shape, creating hot and cold spots in the boiler tubes and causing gas control problems.